A two-phase medium has phase-inversion symmetry if the morphology of phase 1 at volume fraction \(\phi _1\)  is statistically identical to that of phase 2 in the system where the volume fraction of phase 1 is \(1-\phi _1\)  [1]}, i.e.,
\(S_n^{(1)}(\mathbf {x}^n;\phi _1,\phi _2)=S_n^{(2)}(\mathbf {x}^n;\phi _2,\phi _1), \textrm { } n\ge 2.\) 
